The FlatBuffers library in third_party/flatbuffers has some tests missing from the corresponding BUILD.gn.

Some of the tests that are ignored:
 - grpc/tests/grpctest.cpp
 - tests/fuzzer/flatbuffers_parser_fuzzer.cc should probably be tested on ClusterFuzz similarly to [1].

[1] https://cs.chromium.org/chromium/src/testing/libfuzzer/fuzzers/flatbuffers_verifier_fuzzer.cc